Atypical Myopathy

Equine Atypical Myopathy (EAM) is a life threatening equine disease that interferes with the uptake and use of glucose in the muscles. It is extremely serious with a mortality rate of some 75%, early veterinary care is essential for a horse to have a chance of survival.


Recent research, shows the disease to be caused by the toxin hypoglycin A, which can be found in the seeds of sycamore trees (Acer pseudoplatanus) and to a lesser extent in their leaves and it is therefore assumed that the disease is caused by horses eating Sycamore seeds. Other trees of the Acer family may also be implicated, with the box elder tree being responsible for outbreaks of SPM (Seasonal Pasture Myopathy), a very similar disorder that is prevalent in Midwestern USA and Eastern Canada.

The toxin Hypoglycin A affects the body by preventing energy being produced within their muscle cells.

The disease is most common in the autumn when large numbers of seeds are falling, high winds and stormy weather can increase the number of seeds that fall from the trees resulting in an increase in incidence of the disease. Cases of the disease are also seen in spring when Sycamore seedlings may be eaten; however this is unproven as it has not been determined if the toxin is present in the seedlings.

Early recognition and treatment is essential and since the toxin directly targets aerobic energy metabolism, therapy should be targeted at promoting glucose metabolism and provided fluid diuresis.


  • Muscle weakness or stiffness
  • Colic like symptoms
  • Lethargy
  • Laboured breathing
  • Fast or irregular heart rate
  • Dark red-brown urine
  • Choke
  • Recumbency
  • Sudden death


Call a vet immediately if you suspect your horse is suffering from Atypical Myopathy.

Your vet will probably need your horse to be hospitalised, as horses suffering from Atypical Myopathy will need 24/7 nursing and monitoring for digestive and cardiac function.

Fluid therapy is important to help protect the kidneys from damage, horses with atypical myopathy tend to become very dehydrated.

Atypical Myopathy is a very painful condition therefore your vet is likely to administer strong pain relief to help keep your horse comfortable.


  • Avoid fasting
  • The diet should be rich in structural carbohydrate and low in lipids (contradictory to other muscle related conditions) Hi-Fi Senior would make an ideal inclusion in the ration, the blend of high temperature dried grass and alfalfa supplies a very digestible source of fibre but contains no added oil. It is normally a very palatable feed making it ideal to temp a sick horse.
  • Affected horses often develop Hyperlipaemia.
  • Feed small quantities of feed multiple times per day to help avoid peaks and troughs to the glycaemia
  • Allow free access to Fibre Feeds – grass, hay, alfalfa
  • Avoid feeding excess grain
  • B vitamins and antioxidants, such as vitamin E and vitamin C have been shown to be beneficial to help support muscle function. Dengie Performance Vits and Mins and Performance+ Balancer both contain a full range of B vitamins and elevated levels of vitamin E so may be a beneficial addition to the diet.
  • Colic and choke have been associated with Atypical Myopathy, soaked fibre feeds such as Alfa-Beet may be beneficial



Preventative advice for horse owners includes:

  • Supplementary feeding in the field to minimise the risk of horses being tempted to ingest seeds
  • Fencing off affected areas
  • Remove seeds where possible
  • Limiting grass turnout in affected fields
  • Being aware that a field without sycamore trees can still contain seeds spread by high winds or flood water
  • Where a case is suspected, remove field mates from the pasture, blood test to check if they could also be developing the condition. Nutritive support may be beneficial if you suspect a horse may have ingested sycamore seeds.